The present invention relates generally to the field of gas turbine engines, and more particularly to a pilot burner for a gas fired combustor.
FIG. 1 is a schematic diagram of a typical prior art gas turbine engine 10. A compressor 12 draws in ambient air 14 and delivers compressed air 16 to a combustor 18. A fuel supply 20 delivers fuel 22 to combustor 18 where it is combined with the compressed air to produce high temperature combustion gas 24. The combustion gas 24 is expanded through a turbine 26 to produce shaft horsepower for driving the compressor 12 and a load such as an electrical generator 28. The expanded gas 30 is either exhausted to the atmosphere directly, or in a combined cycle plant, is exhausted to atmosphere through a heat recovery steam generator (not shown).
FIG. 2 illustrates one embodiment of combustor 18 where the compressed air 16 and fuel 22 are premixed in a premix section 32 of the combustor 18 upstream of a combustion zone 34 in order to promote a lean, clean-burning, efficient combustion process. Such lean combustion may become unstable, especially during transient conditions. To ensure stable combustion, a fuel-rich diffusion mixture flame 36 may be provided to the combustion zone by a pilot burner 38. Fuel is injected through the pilot burner 38 directly into the combustion zone 34 without premixing with air. Combustion is initiated in the combustor 18 with a pilot flame by providing fuel through a fuel tube 40 that is centrally disposed within the combustor 18. The fuel tube 40 is centrally located within a shell 41. The shell 41 has an inlet end 43 for receiving compressed air 16 and an outlet end 45. A swirl vane 47 may be located proximate the outlet end 45. Fuel tube 40 has an outlet end 42 with a plurality of pilot fuel nozzle openings 44 formed therein for providing a flow of pilot fuel 46 for mixing with the compressed air 16 exiting swirl vane 47. This fuel-air mixture supports the pilot diffusion flame 36. A main fuel flow 22 is provided to the premix section 32 by main fuel nozzles 48 for combusting in the combustion zone 34. The pre-mix combustion is supported by the pilot diffusion flame. Power is increased first by increasing the flow of pilot fuel 46, and then by gradually increasing the main fuel flow 22 as the flow of pilot fuel 46 is decreased. Under full power conditions, the pilot fuel flow rate is decreased in order to reduce undesirable emissions into the atmosphere. Under low power and transient power conditions, the ratio of pilot fuel to total fuel flow may be as high as 50%. Thus, the pilot burner 38 must be designed to deliver enough fuel to produce as much as 30-40% of the rated power level of the engine 10. Since the maximum supply pressure of fuel is fixed, the size of the pilot fuel nozzle openings 44 must be made sufficiently large to ensure that an adequate flow rate of pilot fuel 46 can be provided at the lower power conditions. This opening size then determines the pressure differential across the pilot fuel nozzles 44. Once the pilot fuel flow 46 is decreased at full power conditions, the differential pressure across pilot fuel nozzle openings 44 may become unacceptably low, making the pilot flame susceptible to instability and even flame-out due to relatively minor fluctuations in fuel supply pressure. Since pressure drop is approximately proportional to the square of the mass flow (Mach number less than 0.8), a turndown of 20 to 1 means that the pressure drop at the low pilot flows typical of base load are approximately {fraction (1/400)}th of the pressure drop at the highest pilot flows typical of full power engine operation.
A pre-mix pilot burner design recently developed by Siemens Westinghouse Power Corporation provides a portion of the pilot fuel flow through premix pilot fuel outlet orifices 50 located on swirl vane members 52 upstream of the outlet end 42 of the pilot fuel tube 40. Other geometries associating a premix fuel supply outlet and a swirl member may also be used. This innovation reduces the undesirable emissions produced by a pilot burner 38 by premixing all or a portion of the pilot fuel flow, however, as the diffusion portion of the pilot fuel flow is decreased, the pressure differential across the pilot burner diffusion nozzles 44 is further decreased.
U.S. Pat. No. 5,036,657 issued to Seto, et al., describes a dual manifold fuel system for a liquid-fueled turbofan engine for aircraft propulsion. Two sets of main fuel outlets are provided, with one set always fueled and the second set fueled along with the first set only at higher flow rates. A valve controlling the flow through the second set of outlets is computer controlled, using inputs such as fuel flow, air flow, weight on wheels, and other engine and flight parameters. U.S. Pat. No. 4,499,735 issued to Moore, et al., also describes a liquid fuel injection system for an aircraft gas turbine engine. The main fuel supply to the combustor is divided into a plurality of radially displaced zones and a plurality of circumferentially displaced segments. Fuel distribution is varied across the zones and across the segments during various conditions of operation of the engine in order to provide improved temperature control, fuel atomization, and ignition control. Such complicated fuel system designs are useful for addressing the unique problems presented by the main fuel supply of a liquid-fueled aviation engine, but they do not solve or even address the issue of inadequate pilot fuel nozzle differential pressure in a gas-fueled turbine engine.
U.S. Pat. No. 5,901,555 issued to Mandai, et al., does address the issue of inadequate pilot fuel nozzle differential pressure by dividing the pilot burner diffusion fuel flow into at least two independent systems. The fuel nozzle diameters of each system are different, and each system can be controlled independently. While such a system does provide improved control over the fuel nozzle differential pressure, it is costly to manufacture such redundant systems and more complex to operate an engine containing such independent systems.
Accordingly, a simple, less expensive approach is described herein to assure that adequate differential pressure is maintained across the pilot fuel nozzles of a gas turbine combustor, in particular, a gas turbine having an advanced premixed pilot design. The present inventor has discovered that improved gas-fueled pilot burner performance may be achieved by dividing the diffusion pilot fuel flow into two circuits. A first circuit delivers fuel to a first outlet nozzle, and a second circuit delivers fuel from the first circuit to a second outlet nozzle only when the fuel pressure in the first circuit exceeds a predetermined value. The second circuit may be connected to the first circuit through a spring-loaded relief valve.
In one embodiment, a fuel delivery arrangement for a gas-fueled combustor pilot burner is described as including: a first fuel delivery circuit having an inlet connected to a supply of gaseous fuel and having a first outlet opening disposed in a pilot fuel burner; and a second fuel delivery circuit having an inlet connected to the first fuel delivery circuit and having a second outlet opening disposed in the pilot burner. The fuel delivery arrangement may further include a pressure-regulated valve connected between the first fuel delivery circuit and the inlet of the second fuel delivery circuit for providing gaseous fuel to the second fuel delivery circuit only when a fuel pressure in the first fuel delivery circuit exceeds a predetermined fuel pressure. The pressure-regulated valve may be a spring-release valve.
A pilot burner for a gaseous fuel combustor is described herein as including: a first gaseous fuel delivery circuit having a first outlet disposed within a combustor, a flow of gaseous fuel through the first gaseous fuel delivery circuit being responsive to a fuel flow control system; and a second gaseous fuel delivery circuit having a second outlet disposed within the combustor and having an inlet connected to the first gaseous fuel delivery circuit through a pressure-regulated valve, a flow of gaseous fuel through the second gaseous fuel delivery circuit being responsive to fuel pressure in the first gaseous fuel delivery circuit. The pilot burner may further include a spring-release valve connected between the first gaseous fuel delivery circuit and an inlet of the second gaseous fuel delivery circuit, the spring-release valve adapted to allow the flow of gaseous fuel through the second gaseous fuel delivery circuit only when the fuel pressure in the first gaseous fuel delivery circuit exceeds a predetermined pressure.
In another embodiment, a pilot burner for a gas turbine combustor is described as including: a shell having an inlet end for receiving combustion air and an outlet end for releasing a fuel-air mixture; a premix fuel supply outlet within the shell and a swirl member associated with the premix fuel supply outlet; a first diffusion fuel delivery circuit having an outlet within the shell downstream of the premix fuel supply outlet; and a second diffusion fuel delivery circuit having an outlet within the shell downstream of the premix fuel supply outlet, the second diffusion fuel delivery circuit having an inlet connected to the first diffusion fuel delivery circuit through a pressure-regulated valve. The pressure-regulated valve may be a spring-release valve.
A method of providing fuel to a gas turbine combustor pilot burner is described herein as including the steps of: providing a gaseous fuel supply; providing a first fuel delivery circuit having an inlet connected to the gaseous fuel supply and having an outlet in the pilot burner; providing a second fuel delivery circuit having an inlet connected to the first fuel delivery circuit through a pressure-regulated valve and having an outlet in the pilot burner; controlling the fuel supply to deliver fuel at a first pressure wherein the pressure-regulated valve remains closed and fuel is delivered to the pilot burner through the first fuel delivery circuit and not through the second fuel delivery circuit; and controlling the fuel supply to deliver fuel at a second pressure higher than the first pressure wherein the pressure-regulated valve is opened and fuel is delivered to the pilot burner through the first fuel delivery circuit and through the second fuel delivery circuit. The method may further include the steps of: providing the outlet of the first fuel delivery circuit with at least one outlet orifice; providing the outlet of the second fuel delivery circuit with at least one outlet orifice; selecting the first and second fuel delivery circuit outlet orifices and selecting a pressure-regulated valve opening pressure so that a flame generated by the pilot burner will remain stable when the fuel supply is controlled to deliver fuel at the first pressure and when it is controlled to deliver fuel at the second pressure. The second fuel delivery circuit may be connected to the first fuel delivery circuit through a spring-loaded valve.